How to reproduce:
1. Open the attached "MRTK-EXAMPLES.zip" project
2. Open the "AdaptiveQualityExampleScene" scene
3. Enter the Play mode
-- Editor crash

Reproducible with - 2017.4.0f1, 2017.4.20f2, 2018.3.6f1, 2019.1.0b4, 2019.2.0a5

Stack trace:
(Unity) VRDeviceHoloLens::SetViewportScale
(Unity) VRDevice::SendEventCallback
(Unity) VRDevice::SetRenderViewportScale
(Unity) XRSettings_Set_Custom_PropRenderViewportScaleInternal
(Mono JIT Code) (wrapper managed-to-native) UnityEngine.XR.XRSettings:set_renderViewportScaleInternal (single)
(Mono JIT Code) [C:\buildslave\unity\build\artifacts\generated\common\modules\VR\VRBindings.gen.cs:112] UnityEngine.XR.XRSettings:set_renderViewportScale (single)
(Mono JIT Code) [C:\Users\<username>\Desktop\1125818\Assets\HoloToolkit\Utilities\Scripts\AdaptiveQuality\AdaptiveViewport.cs:65] HoloToolkit.Unity.AdaptiveViewport:OnPreCull ()
(Mono JIT Code) (wrapper runtime-invoke) object:runtime_invoke_void__this__ (object,intptr,intptr,intptr)
(mono) [c:\buildslave\mono\build\mono\mini\mini.c:4937] mono_jit_runtime_invoke
(mono) [c:\buildslave\mono\build\mono\metadata\object.c:2625] mono_runtime_invoke
(Unity) scripting_method_invoke
(Unity) ScriptingInvocation::Invoke
(Unity) MonoBehaviour::HandleNotifications
(Unity) MessageHandler::HandleMessage
(Unity) GameObject::SendMessageAny
(Unity) Unity::Component::SendMessageAny
(Unity) Camera::SendOnPreCullMessage
(Unity) Camera::CustomCull
(Unity) Camera::Cull
<...>